BAHHHHH, I give up, If you go by the 4 hr in between guesses, it's still up for grabs. All someone has to do is say, an elephant's tookus. (Provided they haven't guessed in the last 4 hrs.)
Makes sense that the 4hrs would go away with the addition of a new hint. Would screw anyone that's guessed inside 4 hours at guessing again after a new hint, and that would be lame. Especially since there isn't a set time when a new hint may pop up.